Capacitance switch

ABSTRACT

A capacitance switch in which a capacitance spacing element is bent around an end of a spacer centrally relieved for strain relief.

RELATED APPLICATIONS

This application is a continuation-in-part of pending U.S. patentapplication Ser. No. 237,098, filed Feb. 23, 1981, now abandoned, whichis in turn a continuation-in-part of pending U.S. patent applicationSer. No. 228,118, filed Jan. 26, 1981, entitled CAPACITANCE SWITCH.

FIELD OF THE INVENTION

This invention relates to capacitance-type switches.

BACKGROUND OF THE INVENTION

It is well known in the art to provide capacitance-type switches inwhich a movable key acts to increase capacitance, which change is thensensed for switching action.

SUMMARY OF THE INVENTION

I have discovered that an improved capacitance-type switch can beprovided if an electrically integral capacitance spacing layer isprovided over a plurality of switch zones.

In another aspect of my invention, I had found that membranecapacitance-type switches may be usably and desirably provided.

In preferred embodiments of my invention, I provide at one end of thecapacitance spacer a folded-over portion; I use a screened insulatingepoxy resin as a mechanical spacer; and I use a layer comprising indiumto facilitate electrical connecting.

Devices according to my invention have many advantages. Because the areaof the capacitance spacer layer is increased, the difference betweenunactuated and actuated capacitance is greater, for a greatersignal-to-noise ratio. Furthermore, in its membrane switch aspect thereis provided a sealed capacitance-type switch, protected against dust andvapors. Long life is provided. Because current flows can be madeextremely small, not only can much smaller quantities of metal be usedfor conductors and contacts, but less expensive, non-precious metals maybe used for the most part. Other advantages will appear in the followingdiscussion.

PREFERRED EMBODIMENT

I turn now to a presently preferred embodiment of the invention.

DRAWINGS

There is shown:

FIG. 1 is a plan view of a preferred embodiment of the invention;

FIG. 2 is an enlarged cross-sectional view, taken at 2--2 of FIG. 1;

FIGS. 3A through 3G illustrate steps in the manufacture of saidpreferred embodiment;

FIG. 4 is a circuit diagram of a detection circuit for said preferredembodiment; and

FIG. 5 is an enlarged sectional view, taken at 5--5 of FIG. 1, throughthe joint between the tail and tail extender.

DESCRIPTION

Turning now to the drawings, there is shown a membrane switch (i.e., aswitch in which a flexible layer is flexed in order to produce a signaloutput) in which the signal output is produced by a large change incapacitance.

The device may perhaps be most understandably explained by an initialdescription of how it is made.

There is shown in FIG. 3A a bottom plan view of top layer 10. Itcomprises 5-mil thick transparent polyester layer 10, on the bottomsurface of which has been vacuum deposited thereon through a suitablemask a multiplicity of round pads 12 and conductors 14, of copper, 1500Angstrom units thick.

FIG. 3B is a bottom view of said top layer 10 showing positioned overthe surfaces, including the pads and connectors, illustrated in FIG. 3A,a layer 16 of insulating epoxy (one mil thick), ultraviolet curable andcapable (like the other elements of the switch) of withstanding elevatedtemperature and humidity (e.g., 85° C. and 85% R.H.), screened through amask prior to curing, and including a multiplicity of zones 18 in which,corresponding with the locations of pads 12, the epoxy is laid down, notin an uninterrupted layer as throughout most of the device, but in setsof grid lines 20. The layer of epoxy is also relieved at three areas 21,corresponding with the location of contact areas 22 (shown in FIG. 3A).

FIG. 3C is a plan view of bottom layer 24, also of five mil thicknesstransparent polyester film, on which has been deposited, through a mask,a 1/2-mil thick layer 26 of pressure-sensitive acrylic adhesive,interrupted by generally circular zones 28 corresponding to thelocations of conducting pad zones 22 on the top layer 10.

FIG. 3D is a plan view of the bottom layer 24 following the next step inthe manufacturing process, namely, the assembly of the three elongatedstrips 30, which are held in place by adhesive layer 26, and are formedfrom 1/4-mil polyester on each outer surface of which is vacuumdeposited by RF sputtering layers 1000 Angstrom units in thickness ofaluminum.

FIG. 3E is a plan view of bottom layer 24 after the next step inmanufacture of the device. In this step there is deposited through amask, over the area indicated in dots, a layer 31 of acrylicpressure-sensitive adhesive one mil in thickness. As shown in thedrawings, this step leaves uncoated areas 32 corresponding to thecontact areas 22 in top element 10 and areas 34 corresponding to thepads 12 of element 10.

The step just described leaves extending freely the tips 36 of strips30. Owing to the presence of circular zones 28 in adhesive layer 26,strips 30 are free of adhesive contact through the central portion offolded tips 36. These tips are then folded up around edges 38 ofadhesive layer 31, as illustrated in FIG. 3F, as well as in FIG. 2. Dots40 of a silver filled conductive epoxy (Amicon 3050, sold by AmiconCorporation of Lexington, Mass.) are then deposited along the fold lineof the strips 30, to supplement conductivity through the outer aluminumlayers of strips 30 and make contact with pads 22.

The steps thus far described are carried out on the top and bottomelements (to be assembled into the switch) separately, and while each ispart of a web of greater width than the final width dimension of theswitch. In the top element, portions to define the tail 42 are cut outbefore the upper and lower portions are assembled. Also before assembly,a cut is made to define edge 46 of the lower portion as well as a windowof which the edge 46 is one of the four sides to permit movementtherethrough for soldering of tail 42. They are then assembled, and heldtogether by adhesive layer 31. Next they pass over a roller which causestail 42 to extend in a direction facilitating adding thereto, over theconductor areas 44, by standard solder wave techniques, one mil thicklayer of 50% indium-50% tin alloy. (This approach permits use of a veryshort integral tail, with consequent savings of expensive polyester; aslong a tail as desired may then be attached through an alloy seal with asimilar alloy on the tail extended; the seal can be simply effected by apressure-heat step--50 psi at 125° Centigrade for half a second.) Theconstruction of the tail-extender joint is shown in FIG. 5, in which isseen switch top layer 10 bearing conductive tracks 80 and insulatinglayer 16, extending over switch bottom layer 24. Tail extender 82initially carries copper track 84 protected with epoxy layer 86 and anindium-tin layer over the tracks at their ends to enable making theseal. The indium-tin layer 88 is created by fusion pursuant to pressureof the indium-tin layer carried by tail 42 and the indium-tin layercarried by extender 82. Adhesive strips 90 and 92 cooperate to aid inmaintenance of the seal.

A suitable detection circuit is disclosed in FIG. 4. A single switchshown as indicated at 50 corresponds to one of the 12 switching areasshown in the preferred embodiment. Clock 52 sends out square wave pulsesat a frequency of 2 kHz through inverter 54 and resistance 56 into oneside of switch 50 and simply through resistance 58 into the other. Solong as the switch 50 is not actuated, so that both air capacitor 60 andpolyester capacitor 62 are in the circuit, current flows throughinverter 64 and transmission gate 66 are insufficient to transmitsignals past RC networks 68 and 70. When the air capacitor 60 is takenout of the circuit by pressing the appropriate area, signals do pass RCnetworks 68 and 70, providing output signals at transmission gates 72and 74, the former providing information, in the preferred embodiment,of the X position of the switch pressed, and transmission gate 74providing similar information as to the Y position.

In operation, a pad 12 is pushed downwardly by flexing flexible layer10, which carries it, until pad 12 engages a strip 30. (The amount offorce required to do this may be regulated partially by the spacing ofepoxy grid lines 20; the more closely spaced are the grid bars 20, themore difficult is it to push down layer 10 and pad 12 between them toengage strip 30.) This in effect eliminates one of two capacitors inseries (the air capacitor 60), leaving the polyester capacitor, causingthe capacitance of the circuit through this contact made to increaseenormously. In the preferred embodiment just described the capacitanceincreases from 30 picofarads to 10 nanofarads--i.e., over three hundredtimes.

The freedom from contact with adhesive in the central portions of thefold-overs of strips 30 provide important strain relief during flexingand accordingly improved life.

OTHER EMBODIMENTS

Other embodiments within the invention will occur to those skilled inthe art. For example only, I mention a few. For the polyester layerproducing the large capacitance could be substituted, for example, adeposited layer of tantalum pentoxide, which has a dielectric constantmany times that of polyester. Instead of insulating epoxy a diecutpolyester spacer could be used. Conductor areas could be made byprinting or etching. The polyester capacitor strip could be folded onitself to produce increased capacitance. Instead of being constructed inthe X-Y matrix disclosed the device could be a common ground one, with aseparate lead coming off each conductive pad; here the other lead to allthe pads might desirably be a single large capacitor spacer (as opposedto the three strips shown in the embodiment described). The entiredevice could be made transparent, so that it might be mounted on theface of a CRT. The substrate of the device could be made rigid. Inanother embodiment, the capacitor spacing layer is a single sheetsubstantially the size of the overall device and located in it below alayer carrying on it a plurality of X-row pads and beneath it a layer ofY-column pads, with spacers between the central layer and each of theother two.

OTHER INVENTIONS

The detection circuit disclosed herein is the invention of James P.Walber. The grid system described herein is the joint invention of Dr.Brian E. Aufderheide and the undersigned. Nor was the conception,broadly, of a tail extender, mine.

What is claimed is:
 1. A membrane capacitance switch wherein switchactivation produces a change in capacitance detectable by externalcircuitry, comprising:a substrate sheet, a flexible sheet extending oversaid substrate sheet, a conductive contact carried by one of saidsubstrate sheet and said flexible sheet, a capacitance element locatedbetween said substrate sheet and said flexible sheet and aligned withsaid contact,said capacitance element comprisingan inner insulatinglayer and first and second outer conductive portions on oppositesurfaces of said inner insulating layer, said capacitance elementforming a first capacitor, spacing means for spacing said capacitanceelement from said contact so as to provide an insulating zone betweensaid contact and said first conductive portion,said contact, insulatingzone, and first conductive portion being adapted to form a secondcapacitor electrically in series with said first capacitor, saidflexible sheet, spacing means, contact, and first conductive portion allbeing adapted and located to cause the spacing between said contact andfirst conductive portion to vary when said flexible sheet is flexedduring switch activation and to thereby cause the net capacitance ofsaid first and second capacitors to change to an extent detachable byexternal circuitry, said spacing means including a spacer layeroccupying the space between said flexible layer and said capacitanceelement, and said capacitance spacing element being bent around saidspacer in a fold portion at an end thereof.
 2. The switch of claim 1 inwhich said spacer comprises adhesive material.
 3. The switch of claim 2in which said spacer is centrally semi-circularly relieved adjacent saidfold portion.
 4. The switch of claim 3 in which the transverse edges ofthe end of said spacer including said relieved portion of said spacerengages said fold portion.
 5. The switch of claim 2 in which said spaceris centrally relieved adjacent said fold portion.